The emission wavelength of the semiconductor light emitting devices such as lasers developed to date is mainly in the near infrared (IR). This is limited by the band-gap of the semiconductor material in the active region where the stimulated recombination of electrons and holes across the band-gap gives the emission of electromagnetic radiation. Longer wavelength semiconductor light sources, particularly in naturally occurring atmospheric IR transmission bands (3-5 .mu.m and 8-12 .mu.m), are demanded by many military and civilian applications such as free-space communications, medical diagnostics, atmospheric pollution monitoring, and IR radar for aircraft and automobiles. There have been many attempts devoted to developing long wavelength IR sources by employing intersubband transitions in artificial quantum well (QW) semiconductor heterostructures since an original proposal by Kazarinov et al. in Soviet Phys. Semicond. Vol. 5 (4), 1971. The wavelength of such an IR source due to intersubband transition is determined, not by the band-gap, but by the smaller energy separation of conduction subbands arising from quantum confinement in QW heterostructures made from relatively wide band-gap semiconductor materials. Therefore, the emission wavelength can be tailored over a wide spectral range from mid-IR to sub-millimeter by merely changing QW layer thickness. One recent development in such an intersubband light emitting device is reported by Faist et al. in Science, Vol. 264, pp. 553-556, Apr. 22, 1994 and in Electronics Letters, Vol. 30 (11), 1994, who demonstrate a so called quantum cascade laser which consists of 500 layers, some as thin as 0.8 nm.
A difficulty associated with prior art intersubband light emitting devices is that a high carrier injection efficiency is hard to achieve without reducing population inversion which is essential to lasing action. Such a difficulty would also limit the practical range of wavelength tuning. An issue of more concern for intersubband light emitting devices is that the electron non-radiative relaxation between subbands with energy separation higher than the optical phonon energy (.about.30 meV) is very fast due to optical phonon scattering, and its typical relaxation time (.about.1 ps) is at least three order of magnitude smaller than the radiative time (&gt;1 ns), resulting in a very low radiative efficiency (&lt;10.sup.-3). There is a need for a QW semiconductor IR source having a improved radiative efficiency by suppressing non-radiative relaxation loss, and a high carrier injection efficiency without difficulties and restrictions in practical implementation.